This project consist of two interacting studies. One study focusses on the effect that the optical train of the E-ELT, especially the segmented mirrors, has on the polarisation aspects of the incoming light. Another study looks at a specific optical part, the polarisation modulator. For both studies the goal is to get a thorough understanding of the influence of the optical train on the polarimetric signal.

Approach

The space between the segments of the main mirror of the E-ELT leads to changes in the polarization status of the light reflected from this mirror, which will negatively influence the observing capabilities for instruments like EPICS. It is therefore of the utmost importance that these effects are accurately modelled and verified, so that we can take system level decisions to optimize the combination telescope and instrument. In this project the E-ELT is simulated numerically and the outcome is verified by observations with segmented systems like the Gran TeCan.

This project consist of two interacting studies. One study focusses on the effect that the optical train of the E-ELT, especially the segmented mirrors, has on the polarisation aspects of the incoming light. Another study looks at a specific optical part, the polarisation modulator. For both studies the goal is to get a thorough understanding of the influence of the optical train on the polarimetric signal.

System effects

At this moment there is still no standard method to estimate and predicate errors in polarimetry.

Although there are theoretical arguments, which make it logical that this should be possible, it has never been investigated in enough detail. The main goal of this study is the development of a systematic approach towards a segmented mirror, like the one for the E-ELT, in order to be able to estimate the polarimetric accuracy that can be achieved in such a system. Before this study we did not have any tool to predict the polarimetric performance. By the development of polarimetric performance tool in this study we will be able to significantly reduce the cost of an instrument, by avoiding overspecifications. If successful this tool will show its value in all future instruments that want to study the polarised light.

Polarisation ModulatorThe polarimetric sensitivity of the E-ELT instruments will be determined by a combiation of all optical elements from the telescope up to the detector. One element that has our specific attention is the polarisation modulator en the accompanying demodulating detector. This element is especially important for EPICS. Present activities related to SPHERE-ZIMPOL instrument showed that the polarization modulator is probably the biggest source of errors and that the demodulating detector probably has the largest development risk. Together this is enough reason to spend already now, during the early development of EPICS, manpower and money to this issue.

Approach

System effectsThe space between the segments of the main mirror of the E-ELT lead to polarisation effects in the observed light, and will therefore reduce the observing capabilities of instruments like EPICS. It is therefore of the utmost importance that these effects are well categorized, so that at system level decisions can be taken to optimize the telescope-instrument combination. In order to achieve this new ideas will be worked out and applied to the E-ELT and EPICS design. Afterwards the method will be verified with ExPo on the 4.2-meter William-Herschel Telescope on La Palma and with the existing 10 meter segmented telescopes like the GranTeCan, and SPHERE on the VLT.

Polarisation Modulator

As already outlined above, the errors and development risks for the polarisation modulator and the accompanying demodulating detector are not well understood yet. In this study we will try to develop models, with which we will hopefully better understand the source of the error in the polarisation. The models will be tested bij laboratory experiments. Furthermore we plan to develop new methods to minimize the harmful effects of this component.

Status

The project already showed that the way the data is reduced has a sugnificant influence on the final performance and sensitivity. A factor of 10 improvement can be easily achieved. Furthermore it showed that the influence of the E-ELT on the polarization of the signal is significant but can for a very large part be calibrated out.